Abstract

For the purpose of this investigation, ultra-high resolution bathymetric and marine magnetic data have been collected for the one-nautical-mile zone around Robben Island in Table Bay and synthesised with previously acquired sidescan sonar data of the same area. These data have allowed the offshore resolution of the Late Cretaceous Cape Peninsula – False Bay dolerite dyke swarm to be improved while markedly increasing the bathymetric resolution and aerial extent of available lithological outcrop to which structural trends can be applied.

The Malmesbury Group rocks, which comprise the base of Robben Island, form part of the core of an orogenic belt which is poorly understood owing mainly to the fact that exposures of these rock types are generally sparse and small and/or deeply weathered. The acquired offshore data encompass 28.8 km2 of which ~76% is made up of exposed Malmesbury Group rocks. Previous work on the rock outcrops around the island’s margin (0.6 km2) allowed inferences of the structural and tectonic setting under which these rocks formed. During the present investigation an additional 21.8 km2 of Tygerberg Formation rocks have been mapped, adding to an extremely sparse dataset to aid and complement the work done to date. Multiple phases of deformation in the form of folding and faulting have been preserved in the seafloor outcrops.

The structures charted from this newly acquired data and their associated trends are consistent with and reinforce the conclusions drawn from the onland studies namely, that there exists a pervasive northwest – southeast regional fabric formed by oblique transpression and metamorphic alteration of marine sediments in a deepening fore-arc basin.

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